Landfills and sewage treatment plants contain siloxanes from many sources, both industrial and domestic. One source of siloxanes is the semiconductor industry, which produces siloxanes as by-products of reactions involving silicon compound gases. Because siloxanes have detrimental effects on semiconductor products, siloxanes are removed from semiconductor process gases by processes such as adsorption onto diatomaceous earth, silica gel, molecular sieves, activated carbon or activated alumina.
The personal care industry uses volatile methyl siloxanes in products such as deodorants, tooth-pastes, skin care preparations, hair conditioners and anti-perspirants.
The cleaning industry finds many applications for siloxanes. In dry cleaning, siloxanes are used as a more environmentally friendly solvent than traditional chlorofluorocarbons. In the electronics industry, siloxanes are used to clean circuitry.
Siloxane-containing waste from industrial and domestic sources is discharged into landfill sites and sewage treatment plants, along with a variety of biological organic matter. The organic matter in the waste decomposes to produce bio-gas containing various volatile organic compounds, such as methane. The bio-gas can be used to fuel various combustion engines to produce power, or both heat and power. However, the bio-gas from landfill sites and sewage treatment plants is contaminated with siloxanes. When an engine burns siloxane-contaminated bio-gas, the siloxanes, on oxidation, forms precipitates of silicon dioxide. The precipitates are deposited on engine parts such as turbine blades, pistons, cylinders, heat exchangers and emission control equipment. The deposits increase the abrasion of engine surfaces, leading to a loss of engine efficiency and premature engine failure. The deposits also poison catalytic converters in emission control equipment.
According to the EPA's most recent data (2007), the U.S. has over 1,700 active landfills. Though the number of landfills has significantly decreased over the last 20 years, the average size of landfills has increased. Landfill sites produce methane and carbon dioxide gases due to the natural decomposition of solid waste material. Solid waste landfills are the second largest source of human-related methane emissions in the United States, accounting for approximately 23 percent of these emissions in 2007. In fact, these methane emissions from landfills represent a lost opportunity to capture and use a significant energy resource. Instead of allowing landfill gas (LFG) to escape into the air, it can be captured, converted, and used as an energy source. Financial benefits and improved community relations now provide the landfill industry with multiple incentives to employ bio-gas conditioning systems in the management of these gases.
Similarly, approximately 14,000 wastewater treatment facilities (WWTFs) operate in the United States, ranging in capacity from several hundred million gallons per day (MGD) to less than 1 MGD. Roughly 1,000 of these facilities operate with a total influent flow rate greater than 5 MGD, but only 544 of these facilities employ anaerobic digestion to process the wastewater. Moreover, only 106 WWTFs utilize the bio-gas produced by their anaerobic digesters to generate electricity and/or thermal energy. If the remaining WWTFs were to install combined heat and power technologies, approximately 340 MW of clean electricity could be generated, offsetting 2.3 million metric tons of carbon dioxide emissions annually. These reductions are equivalent to planting approximately 640,000 acres of forest, or the emissions of approximately 430,000 cars.
Utilization of bio-gas conditioning systems provides landfills and WWTFs with an opportunity to collect and dispose of the high levels of methane found in landfill and WWTF digester gases. Currently, many landfills and WWTFs are using untreated gas containing impurities such as sulfur, chlorine, silicon and moisture, to generate power and fire boilers. This untreated gas can make existing equipment such as boilers, engines, fuel cells and turbines susceptible to increased damages, increased maintenance costs and shorter life spans.
Siloxane concentrations are generally higher in digester gas than in landfill gas. Table 1 shows the various siloxanes encountered in bio-gas facilities. Siloxanes in digester gas appear to be predominantly D4 and D5, while landfill gas may additionally contain other siloxane compounds, like D3 and D6, as well as L2 through L5. In addition, significant quantities of trimethyl silanol can also be present in these gas streams.
TABLE 1MolecularVapor PressureWeightat 25° C.NameFormulaAbbreviations(gm/mole)(mm Hg)HexamethylcyclotrisiloxaneC6H18O3Si3D322210OctamethylcyclotetrasiloxaneC8H24O4Si4D42971.3DecamethylcyclopentasiloxaneC10H30O5Si5D53710.4DodecamethylcyclohexasiloxaneC12H36O6Si6D64450.02HexamethyldisiloxaneC6H18Si2OMM, L216231OctamethyltrisiloxaneC8H24Si3O2MDM, L32363.9DecamethyltetrasiloxaneC10H30Si4O3MD2M, L43100.55DodecamethylpentasiloxaneC12H36Si5O4MD3M, L53840.07
The landfill gas and wastewater digester gas industries, when generating power, have been dealing with the impact of siloxane build up for years. Increased equipment maintenance and replacement costs due to siloxanes from wastewater digester gas systems and landfill gas systems were assumed to be standard operating issues that each plant was forced to incorporate into their operating budgets. Attempts to minimize environmental pollution, specifically NOx reduction technologies, such as selective catalytic reduction to reduce air emissions from bio-gas fueled equipment, often resulted in failure of the catalyst after a few days due to the presence of siloxanes, with the catalyst surfaces getting coated with a thin layer of silicon dioxide and thereby rendered inactive. Biogas with siloxanes higher than 50 parts per billion by volume (ppbv) can cause significant damage to gas turbine power generation equipment. Levels higher than 100 ppbv may also damage internal combustion engines. The proper treatment of input gas using bio-gas purification technologies needs to remove siloxanes to levels of less than 10 ppbv.
Allowable siloxane contents for various power generation turbine systems are shown in Table 2, below, as obtained from various turbine manufacturers. Typical landfill gas siloxane contents range around 20-35 mg/m3, and thus are much higher than allowable limits in most turbine fuel specifications.
TABLE 2Turbine manufacturersSi limits in mg/m3Capstone Turbines0.03(5 ppbv)Solar Turbines0.1 (16.7 ppbv)Ingersoll Rand (FlexFuel)0.06 (10 ppbv)GE Jenbacher10GE Waukesha25Deutz5Caterpillar Turbines28
Siloxane removal systems have historically taken the form of chilling or adsorption/absorption. Some systems incorporate moisture control with chilling to −7° C., essentially to remove the volatile siloxanes by manipulation of their dew points. The most widely used method to reduce siloxane concentrations is adsorption on activated carbon (or activated alumina). Depending on the activated carbon, overall silicon reduction of up to 98% is possible. Since bio-gas contains a broad range of different compound classes with concentrations varying over several orders of magnitude, competitive adsorption of contaminants occurs. The presence of relatively non-volatile, sulfur-containing or halogenated compounds, for example, can greatly reduce the adsorption capacity of the activated carbon towards siloxanes. Other factors influencing the siloxane adsorption capacity of activated carbon are the relative concentrations of the siloxane species to one another, alas well as temperature and relative humidity. Similarly, activated alumina can also be used for siloxane adsorption. Regeneration of the adsorption media is accomplished by passing hot, inert gases through the media to desorb the siloxanes and vent them into the atmosphere. Siloxanes are not considered volatile organic compounds (VOCs), and thus can be discharged into the atmosphere, where they oxidize with atmospheric oxygen to silicon dioxide over time.
Sites such as the Toland Road Landfill in Santa Paula, Calif., require a monthly change of the activated carbon, once saturated with organic and other contaminants. The cost for regeneration of activated carbon adsorbents is approximately $10,000 per change at the Toland Landfill Site. Regenerating activated alumina is cheaper than regenerating activated carbon, though the initial costs of activated alumina are higher.
Absorption technologies are another pathway for siloxane removal. Physical and chemical absorption can be distinguished. In theory, the latter is applicable to siloxane elimination, as the siloxanes are destroyed by strong bases and acids at high or low pH-values, respectively. However, the potential application of these chemical absorption agents is associated with high costs, safety and corrosion issues, and therefore has not been adopted widely. Moreover, only acidic liquids can be used for bio-gas facilities, unless carbon dioxide is removed upstream of the siloxane removal system for fuel gas upgrading, as bases react with the carbon dioxide in the bio-gas to form carbonates, which in turn precipitate onto the equipment. Among the most effective acidic solutions are nitric acid (>65%) and sulfuric acid (>48%), which remove many species by over 95%. This is only possible at temperatures above 60° C., while elimination at 20° C. is noticeably lower.
The second type of absorption is physical, including absorbents such as water, organic solvents or mineral oil. The absorption of volatile methyl siloxanes (VMS), most of which are hydrophobic, in water (at pH 7) was not proven to be very successful; nevertheless, some water-soluble contaminants including trimethylsilanol and hydrogen sulfide could be removed. Absorption using water is therefore a common pre-conditioning step to enhance the effectiveness of subsequent adsorption by activated carbon. Absorption with liquid solvents like ‘SELEXOL’ (essentially composed of polyethylene glycol dimethyl ether, PEGDME) from Dow Chemicals has also been used for removal of siloxanes, though the solvent is primarily used for removal of acidic gases like carbon dioxide, hydrogen sulfide, COS and mercaptans. These species dissolve appreciably in the solvent before the siloxanes are solvated. Regeneration of SELEXOL is accomplished by scrubbing the solvent with hot inert gases or by exposure to high temperatures, whereby the contaminants volatilize out, and leave the solvent behind for reuse and recycling into the liquid scrubbing system. There are many different liquid chemicals that are able to preferentially absorb contaminants from landfill gas. An absorption column could be designed around any one of these chemicals and the result would be efficient removal of siloxane contaminants. The problem with physical absorption is that once the absorption media reaches saturation, regeneration is required. This can be cost prohibitive in the case of the aforementioned liquid absorbents, if done using traditional methods like subsequent vaporization of the volatile contaminants. Water is not a suitable solvent for most siloxane compounds. Many different organic compounds such as methanol and ethanol also do not function quite well as siloxane solvents. Most siloxanes are non-polar compounds, and hence the use of polar solvents like water, methanol and ethanol, or glycols like ethylene glycols and propylene glycols, provides insufficient solvation of the siloxane species, rendering them unsuitable for dissolving the siloxanes for subsequent removal or sequestration.
Another known technique is condensation of the volatile siloxanes under high pressure but low temperatures. However, mere cooling to temperatures around 5° C. has proven to be unsuitable for quantitative VMS removal. The removal efficiency achieved by deep chilling depends on the respective siloxane concentrations in the raw gas. The more volatile the siloxane moiety, the more difficult it is to condense; certain troublesome species can practically not be reduced to concentrations lower than present in bio-gas. In order to reach higher removal efficiencies, condensation could theoretically be performed at higher pressures. Attainable siloxane concentrations decrease by the same factor as the operating pressure is increased. It is clear that the lower the siloxane load of the raw gas, the more difficult it is to remove siloxanes. A landfill gas with silicon concentrations of 50 mgSi/m3 must be chilled to −30° C. to reach a removal efficiency of roughly 50%, while −40° C. would remove roughly 70%. Still lower temperatures do not lead to any more significant purification, as the remaining siloxanes do not condense noticeably. Only at unusually high siloxane loads and very low sub-zero temperatures does condensation of these species commence. Due to relatively high investment and operating costs, deep chilling is generally regarded as economically suitable only at high gas flow rates and elevated siloxane load. In addition, the process is also subject to problems of ice formation of any contaminating water present. New siloxane removal methods being investigated include temperature swing absorption (TSA) or pressure swing absorption (PSA), whereby temperatures and pressures, or both, are manipulated, along with use of adsorbents, to remove the volatile organics by simultaneous condensation and adsorption.
One method currently being investigated is the use of gas separation membranes. This implies the principle of selective siloxane permeation by solution and diffusion through dense polymeric membrane materials. Ideally, the product component methane needs be retained as much as possible and should not pass through the membrane. Hybrid membranes comprised on polydimethylsiloxane (PDMS) and zeolites have been proposed in literature, though VMS cleanup has not been very successful.
Thus, previous attempts at removing siloxane contaminants from bio-gas have used adsorbents such as activated charcoal, activated alumina, molecular sieves and silica gel, or chilling and refrigeration techniques to condense and separate the volatile siloxanes from bio-gas. However, none of these techniques are able to clean up siloxane contamination at low costs. Improved techniques for removing siloxanes from bio-gas are thus needed, but has eluded persons of ordinary skill in the art. The present invention solves this problem, as described below.